The present invention relates to the field of embedded inductors and, more particularly, relates to tunable, three dimensional embedded inductors buried in a dielectric substrate, most preferably a multilayer ceramic (MLC) substrate.
Conventional MLC structures are formed from ceramic greensheets which are prepared by mixing a composition of ceramic particulate, a thermoplastic polymeric binder, plasticizers and solvents. The ceramic particulate may contain particles of, for example, alumina, aluminum nitride, glass-ceramic, glass plus ceramic, and silicates. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible greensheets. After blanking, via formation, screening of electrically conductive vias and lines, stacking and laminating, the greensheet laminates are eventually fired at temperatures sufficient to drive off the polymeric binder resin and sinter the ceramic particulates together into a densified ceramic substrate. The metals used for the electrically conductive vias and lines are chosen to be compatible with the ceramic material and may include copper as well as refractory metals such as molybdenum and tungsten.
MLC structures are not widely used for radio frequency and analog devices such as rf amplifiers, transformers and impedance matching networks because there have not been easy, cost effective ways to integrate inductors, transformers and the like into the MLC structure. Q is a quality factor for inductors and is defined as the ratio of its reactance to its effective series resistance at a given frequency. It would be desirable to have an embedded inductor with a relatively high Q value and high inductance.
Others have proposed various embedded inductors.
Muckelroy U.S. Pat. No. 3,812,442, the disclosure of which is incorporated by reference herein, discloses a three dimensional inductor in which a coil is imprinted on each layer. Such a design leads to undesirable capacitance between the layers. Capacitance is undesirable because it decreases self resonant frequency. Further, due to the thinness of the metalization, there is a high resistance path resulting in lower Q.
Fleming et al. U.S. Pat. No. 5,389,428, the disclosure of which is incorporated by reference herein, disclose a process for making surface mount inductors having ferrite cores.
Hwang et al. U.S. Pat. No. 5,610,569, the disclosure of which is incorporated by reference herein, disclose an embedded inductor formed of conductive strips and columns of vias. Due to overlap of the top and bottom portions of the inductor, undesirable capacitance results along with lower Q.
Lipkes et al. U.S. Pat. No. 5,945,902, Sasaki et al. U.S. Pat. No. 6,008,151, Kumagai et al. U.S. Pat. No. 6,147,573 and Takeuchi et al. U.S. Pat. No. 6,189,200, the disclosures of which are incorporated by reference herein, disclose embedded inductors having one turn of the coil per each layer resulting in high turn to turn capacitance.
Alford et al. U.S. Pat. No. 6,008,102, the disclosure of which is incorporated by reference herein, disclose a three dimensional inductor coil fabricated on top of a semiconductor substrate. It is noted therein that the insulating core of previous prior art devices is not favored because it is too lossy for many high frequency applications. Further, the inductor structure has very thin conductor lines which lead to high resistance and low Q.
Yamamoto et al. U.S. Pat. No. 6,104,272, the disclosure of which is incorporated by reference herein, disclose a coil which is subsequently embedded in a ceramic chip element.
Libertore et al. U.S. Pat. No. 6,160,469, the disclosure of which is incorporated by reference herein, disclose a two dimensional inductor. Two dimensional inductors are not preferred because three dimensional inductors have less conductor length per a given volume. Further, two dimensional inductors are more affected by outside fields because of the large, unprotected fringe field whereas in three dimensional inductors, a large portion of the fringe field is located within the loops which shield it.
IBM Technical Disclosure Bulletin, 29, No. 2, p. 783 (July 1986) discloses an embedded MLC coil which is used as a magnetic deflection coil for an electron beam lithography machine. Such a structure cannot be integrated in an electronic package.
Notwithstanding the above-noted work of others with respect to embedded inductors, there remains a need for an improved embedded inductor, particularly one buried or embedded in MLC.
Accordingly, it is a purpose of the present invention to have an embedded inductor having high Q and high inductance.
It is another purpose of the present invention to have an embedded inductor with low capacitance.
It is yet another purpose of the present invention to have an embedded inductor that is tunable.
These and other purposes of the present invention will become more apparent after referring to the following description considered in conjunction with the accompanying drawings.